1. Field of the Invention
This invention relates to a protection circuit for an output transistor for example in an audio power circuit.
2. Description of the Prior Art
Usually, the output transistors providing the output signals in electric circuits or devices are susceptible to problems caused by the noise of external circuits or power supplies, and short circuiting problems.
Therefore, up to now, many kinds of protection circuits for the output transistors are used.
But the conventional protection circuits are operable by detecting the collector current or collector to emitter voltage of the output transistors. When the detected current or voltage exceeds the rated current or voltage of the output transistor, the power supply for the output transistor is cut by the operation of the protection circuit, for instance.
As described above, in the conventional protection circuits, only one of current or voltage of the output transistor is detected, such that it is impossible to protect the output transistors widely over the A.S.O. (Area of Safe Operation) of the output transistor.